1. Field of the Invention
The present invention relates to a method and apparatus for forming pictures having gradient from picture information having gradient.
Although the present invention is applicable to various types of picture-forming apparatus such as ink jet printer, electrostatic printer and thermal transfer type printer, the present invention will be described in this specification mainly in connection with an ink jet printer.
2. Description of the Prior Art
In the art of ink jet printer there have been proposed and used the following methods to reproduce the gradient of a picture to be printed.
The first one of the known methods is to reproduce the gradient by changing the volume of ink jetted from the ink jet nozzle so that the difference in dot diameter represents the gradient.
The second one is to reproduce the gradient employing the known dither process. In this method, a picture element is composed of a matrix consisting of, for example, 4.times.4 minute picture elements.
According to the first method, only several levels of gradation can be represented. The reason for this is that it is difficult to obtain a broad range of printable dot diameters from the minimum to the maximum.
The second method is a method proposed to solve the above problem involved in the first method. According to the second method it is possible to reproduce seventeen gradient levels if a picture element is composed of a 4.times.4 matrix. However, since one picture element is composed of 4.times.4=16 minute picture elements, the printing speed is decreased down to 1/16 as compared with the first method. In order to maintain the same high speed as that in the first method, it is required to increase the number of printing heads by 16 times. Use of such many printing heads makes the construction and arrangement of heads very complicated. Furthermore, a very large electric circuit is required to carry out the picture processing according to the dither process, which results in large cost increases. Another drawback of the second method is found in that the picture element composed of a matrix is too large to give a good print. The minimum diameter of a printable dot is limited. At present the smallest possible dot diameter is about 70 .mu.m. When a picture element is composed of a 4.times.4 matrix using the minimum dot diameter, the size of the picture element is 280 .mu.m-400 .mu.m in length of one side. This size of picture element may be acceptable for a large picture print. However, for those prints which man usually views at the distance of distinct vision (25-30 cm), this size of picture element is too large.
If one wishes to obtain a sufficiently broad gradient range with a smaller picture element, it is required to reduce the size of every dot and also to increase the color density thereof. However, if gradient of relatively low level is represented by such dots each having a small area and high color density, then the printed picture looks rough and unnatural. When one looks at the picture, he has a feeling that the picture lacks repose.
The "unnaturalness" of the obtained print becomes very remarkable in particular in representations of the highlight portion of picture or in representations of human skin. Generally there is difficulty in smoothly reproducing halftones over a broad range.
In an attempt to attain better reproduction of gradient, it can be considered to use inks having different densities in a color.
Where two inks different in density are used according to this method, a picture portion having a density of optical reflection in a certain definite area (hereinafter the density of optical reflection in a certain definite area is referred to as the average density of optical reflection) can be represented in two different manners while using a desired dot pitch (pitch at which the dots are arranged). One is to form small diametered (small dot area) dots by dark or deep (higher color density) ink. Another is to form large diametered dots by light or pale (lower color density) ink. Of course, the use of two inks, dark and light, should be understood as mere example of useful combinations of inks according to the method. Three or more inks different from each other in density and also in color may be used in the same manner.
However, this recording method also has some disadvantages. Even when the two pictures formed in the above two different manners have substantially the same average density of optical reflection, they look different from each other to human eyes when the produced pictures are viewed. The degree of difference between the two pictures which is belt varies depending on the ink density and the dot pitch. However, generally speaking, the picture reproduced by high density and small diametered dots gives a stronger impression of "roughness" and feels much more unnatural to human eyes. In representations of gradient of common pictures, therefore, naturalness has often been lost from the pictures by use of such recording method.
Also, an attempt has been made to represent the lowest density (the lowest gradient level) of picture by not forming dots there. However, in this case, there is produced in the image a portion left white, the tone of which differs with the tone of other portions where dots have been formed which gives it a heterogeneous tone. Therefore, this method also has resulted in poor quality of picture.